Microelectromechanical system (MEMS) tip and tip/tilt mirrors are commonly implemented in optical switching systems. By changing the angle of each tilt mirror, beams emitted from one optical fiber can be routed to different output optical fibers held in an array. These MEM tilt mirrors are being deployed in small port count switches, such as two-by-two (2xc3x972) switches and larger switching fabrics having hundreds to thousands of ports.
Another common optical component is the tunable Fabry-Perot filter. These devices comprise two or more mirrors that define an optical cavity. They function as tunable band pass filters.
In the past, bulk Fabry-Perot filters have been constructed typically using piezoelectric actuators. More recently, MEMS Fabry-Perot (FP) filters have become increasingly common.
MEMS FP filters are usually manufactured with one of two cavity configurations. The most common implementation presently appears to be, what is termed, a curved-flat cavity. In this implementation, one of the mirrors is substantially flat and the other is curved. The advantage of this implementation is that high finesses can be achieved even with imperfect parallelism between the mirrors. The disadvantage is, however, that in most applications, the beam launch conditions and/or cavity must be carefully designed to avoid the excitation of higher order transverse modes in the tunable filter. The other configuration for MEMS tunable filters is termed a flatxe2x80x94flat cavity. In this implementation, both mirrors are substantially flat. Typically, the finesse in these filters is lower due to beam walk-off because two mirrors may not be being parallel with respect to each other. To further mitigate this problem, larger beam sizes are some times used. The flatxe2x80x94flat cavities are intrinsically single mode, however.
Bulk Fabry-Perot filters have been proposed that have the capability for hitless tuning. During this operation, one of the mirrors is intentionally tilted out of parallelism with respect to the other mirror. The mirror is then translated to modulate the optical cavity. Once the mirror is tuned to the desired location, it is tilted back into parallelism with the other mirror. This functionality allows the filter to tune from one location in the spectrum directly to another location in the spectrum. Only when it is tuned back into parallelism does the cavity resonate to enable the transmission at the passband.
A number of applications for hitless Fabry-Perot filters can be identified. They can be used as tunable detectors and tunable add/drop devices. For example, a WDM signal could be directed at the Fabry-Perot filter, with the reflection from the Fabry-Perot filter being acquired either using a circulator or angle. The tunable Fabry-Perot filter could then be used to separate out individual or multiple channels in the WDM signal for transmission through the tunable filter either to a detector or to another optical fiber link. Bulk Fabry-Perot filters, however, have poor stability due to the use of piezoelectric actuators. Moreover, they are large, making them incompatible with many commercial WDM optical systems.
The present invention is directed to a Fabry-Perot tunable filter. It specifically concerns a MEMS implementation. Moreover, it has the capability for hitless tuning, being tunable between bands directly, i.e., without dropping intervening bands. This is accomplished by using tilt mirror technology, in which at least one of the mirror structures of the tunable filter has the capability of being tilted relative to the optical axis, translated to adjust the optical length of the cavity, and then tilted back into parallelism with the other mirror structure. This modulation of the mirror structure can be accomplished using electrostatic actuation. The MEMS implementation yields a very small size, while the electrostatic operation has very good long-term stability.
In general, according to one aspect, the invention features a tilt mirror tunable filter. It comprises a frame and a first mirror structure. A tether system connects the first mirror structure to this frame. The tether system is designed to enable the tilting of the first mirror structure relative to an optical axis, in addition to translation of the first mirror structure along the optical axis. A second mirror structure is further provided to define an optical cavity in combination with the first mirror structure. At least two drive electrodes are provided for electrostatically tilting and translating the first mirror structure.
In one implementation, a support, such as a handle wafer, is provided. A device layer is then formed, e.g., deposited or bonded, to the support. The frame and the tether system are fabricated in this device layer. The tether system is released from the support to suspend the first mirror structure above the support in a MEMS release process. This is accomplished through the removal of a sacrificial structure from between the device layer and the support, in one example. The sacrificial structure, in one example, can be simply an oxide layer. In a more complex implementation, it can include silicon and oxide layers.
In another implementation, the tether system and frame are manufactured using micromachining techniques in which a device layer is patterned and then lifted from the support using micromachining techniques and/or actuators such as comb drives, for example.
In the preferred embodiment, the support comprises an optical port, which is formed through the support along the optical axis. This provides a device with a low insertion loss since absorption in the support is avoided. Further, according to the preferred embodiment, the mirror structures comprise thin film dielectric mirror coatings. Such coatings enable high reflectivities, such as greater than 95% to greater than 99%, with obtaining low loss. Such mirror coatings are critical to the manufacture of high performance devices.
In one implementation, both the first and second mirror structures are substantially flat. This allows the optical cavity to be easily spoiled by the tilting of one of the mirror structures.
In an alternative embodiment, however, one of the mirror structures has some curvature. The sag, however, of this mirror structure is limited, so that the mirror structures can be tilted with respect to each other adequately to spoil the cavity during tuning or other reflective state.
In general, according to another aspect, the invention features a method for hitless tuning of a MEMS tilt mirror tunable filter. This method comprises controlling electrostatic fields between at least two fixed electrodes and a first mirror structure. This mirror structure is connected to a frame by a tether system, for example. These electrostatic fields are controlled to tilt the first mirror structure relative to a second mirror structure and thereby spoil an optical cavity defined by the first mirror structure and the second mirror structure. Then, with the cavity spoiled, the electrostatic fields between the fixed electrodes and the first mirror structure are controlled to translate the mirror structure relative to an optical axis of the optical cavity. This results in a change in length of the optical cavity. Once the desired length is achieved, the electrostatic fields are then controlled to tilt the first mirror structure relative to the second mirror structure to realign the optical cavity.
The above and other features of the invention including various novel details of construction and combinations of parts, and other advantages, will now be more particularly described with reference to the accompanying drawings and pointed out in the claims. It will be understood that the particular method and device embodying the invention are shown by way of illustration and not as a limitation of the invention. The principles and features of this invention may be employed in various and numerous embodiments without departing from the scope of the invention.